1. Field of the Invention
This invention relates to an automatic transmission, particularly to an automatic transmission for an automobile which is used together with a torque converter, and more particularly to an automatic transmission which can be easily modified to comprise a transmission stage or a multitransmission stage.
2. Description of the Related Art
Heretofore, a four speed automatic transmission mechanism employs a three speed automatic transmission mechanism including two planetary gear units as a base, and an additional overdrive (O/D) mechanism attached thereto or underdrive (U/D) mechanism comprising a planetary gear unit in order to obtain a transmission stage having four foward speeds.
In recent years, in view of ever growing of F F (front engine front drive), high horse power, etc. for vehicles for example, the automatic transmission is required to be much smaller. In addition, the transmitting torque capacity is required to be larger. However, the conventional four speed automatic transmission mechanism including the two planetary gear units attached with the aforementioned O/D mechanism, etc. can not meet the above mentioned requirements.
Under the circumstances, there is an automatic transmission mechanism which additionally has two clutches attached to the conventional so-called Simpson type planetary gear unit for obtaining four speeds proposed in Japanese Patent Laid-open Application No. 59(1984)-183147. That is, the mechanism of the automatic transmission is constituted such that a third clutch is disposed between the sun gear of the first planetary gear unit and the sun gear of the second planetary gear unit which are integrally connected with respect to each other in the conventional Simpson type. At the same time, the sun gear of the first gear unit communicates with the carrier of the second gear unit through a fourth clutch, the third clutch is disengaged to separate the first gear unit from the second gear unit, the fourth clutch is connected in order to transfer input to the carrier of the second gear unit, and at the same time, the sun gear of the second gear unit is fixed to output an overdrive from the ring gear to obtain a fourth speed in addition to three speeds.
On the other hand, in recent years, because many kinds of vehicles are produced and a wide variety of attachments ae made available for one kind of vehicle, various kinds of automatic transmissions are required to be produced. And, there is a tendency to produce many kinds of automatic transmissions but each only in a small quantity. By the way, the four speed automatic transmission apparatus attached with the O/D mechanism or U/D mechanism is entirely different from an apparatus comprising a three speed automatic transmission mechanism in its outer configuration. Accordingly, the whole transaxle case is required to be made differently. This means that although the three speed automatic transmission mechanism is same, it is required to be assembled on an entirely different line. Also, in the four speed automatic transmission which is an improvement of the aforementioned Simpson type, both the sun gears are separated and a clutch is interposed therebetween. Accordingly, all parts including the casing must be constituted differently from the conventional three speed Simpson type, and the three speed automatic transmission and four speed automatic transmission must be manufactured separately. Due to the foregoing, common use of parts and of an assembly line are difficult to attain, and small production of many kinds of parts often invites a significant increase in cost, thus the requirements of recent time demands for various kinds of vehicles and wide variations thereof are unable to be met.
Further, due to the diversity of needs and the demand for decrease of fuel cost in recent times, the appearance of a transmission having many more transmission stages is awaited. Any attempt to cope with the demand for a multitransmission stage results in an extremely large amount of equipment investment.